I. Field of the Invention
The present invention relates to an inspecting device for print for detecting a defect occurred on the printed sheet by comparing the state of the printed sheet during printing with a reference state in a printing machine.
II. Description of the Prior Art
Heretofore, printed sheets have been inspected one by one by human checker in an off-line. This is because even printed sheets of the same pattern are slightly different due to replenishing ink, changes in the ambient temperature, minor troubles in the printing machine, etc. Only the human visual sense can detect such delicate differences.
On the other hand, various methods have been invented to inspect sheet while they are being printed. In one of them, a strobo is used to illuminate the sheets in synchronism with the printing speed. In another method, a mirror is rotated at high speed, thus applying light onto each sheet. Either method provides a stationary image. The apparatuses employed in these methods cannot inspect the printed sheets. The printed sheets need to be inspected by human checkers. Another trial for inspecting color patches which are printed simultaneously with the picture pattern on the sheet instead of inspecting the entire print area of printed sheet have also been proposed. However, according to this method, when a printing defect such as dripped oil, or stains arise on the print pattern of the printed sheet, the defect might be overlooked, and the function of the inspecting device cannot be sufficiently performed.
A system for inspecting the entire print area of a printed sheet in a sheet-fed press, a rotary press and a rewinder, as disclosed in U.S. Pat. No. 4,488,808 entitled "PRINT INSPECTING DEVICE", by utilizing an optical detecting device provided in the press, has recently been proposed. Since the print pattern of the printed sheet can be automatically inspected in line by utilizing this system, the above-mentioned defect is eliminated, and excellent advantage of the system can be expected.
However, this system still has some problems. As one of the large causes, there is a phenomenon that the color density of a printed sheet entirely varies even in the normal operation as the characteristics of a printing machine. In this system for merely comparing the color density of the print with reference information, it is difficult by this system to judge such defects of the printed sheet. The entire variation in the color density is caused by an inking mechanism, the irregularity in the water supply amount by a damping unit and alteration in the ink amount of an ink fountain, and it is very difficult to suppress such variations in the color density within a predetermined value.
Further, a printed web to be inspected is affected by the influence of variations in a tension, feeding velocity and drying temperature, so that the web is fed in a web rotary press to cause a fine speed variation in the feeding direction. Thus, it is very difficult to accurately synchronize a source of inputting by an optical detecting device an image signal in the feeding direction. The difficulty of the accurate synchronization means different pixels to be inspected in the respective picture patterns of the printed web, and an accurate inspection cannot be fundamentally achieved in the system for inspecting a defect by comparing the print with reference information for each pixel.
Moreover, a printed sheet to be inspected is printed with black, cyan, magenta and yellow ink printing in the ordinary color printing. Even when a printing defect occurs only in one of these colors, the inspecting device must have the function of detecting the defect.
In order to solve this problem, a device for separating a signal light from the printed sheet into wavelength bands by utilizing three color separation filters of R (red: wavelengths of 600-700 nm), G (green: wavelengths of 500-600 nm), and B (blue violet: Wavelengths of 400-500 nm), respectively inputting the three wavelength bands into three CCD cameras, inputting the three wavelength bands into a CCD camera by rotating the R, G and B filters to process the R, G and B signals in a time series, or alternatively flashing light sources of R, G and B to provide similar effects has been proposed so far.
However, according to the above-mentioned conventional inspecting device, there are various drawbacks in practical use such that, though a highly inspecting accuracy can be attained, the device requires a large scale memory capacity, increases in size, cost and a complexity of control system.